Superalloys for the Energy Sector, Part 3 - Molten Salt Reactors and Hastelloy® N superalloy

In the United States, first commercial project of molten salt reactor (MSR), classified as Gen IV nuclear reactors, is implemented. These reactors are intended to be a safer and more environmentally friendly alternative to older Gen II and Gen III, water-cooled reactors. Hastelloy® N superalloy was developed specifically to deal with the challenging corrosive environment inside the reactor and the cooling system. What makes Hastelloy N so special? Why MSR's require a tailored superalloy?

Molten Salt Reactors

Molten Salt Reactors (MSR) use liquid salts, usually a mixture of uranium, sodium fluoride, and zirconium, for fuel. When the fuel gets into the reactor it, well, reacts, generating outstanding amounts of thermal energy. As the fuel salt exits the reactor, this heat is transferred through heat exchangers to a secondary cooling circuit, where another, non-radioactive salt mixture flows. The heat is subsequently transferred to the turbine drive circuit. The use of hot, liquid salts as both fuel and coolant offers several advantages over water-cooled reactors:

• Higher temperature, lower pressure - salts remain liquid at temperatures of 600–900°C without the need to increase the pressure. In contrast, in water-cooled reactors, water needs to be pressurized to whooping 150 bars to stay liquid - and even then it can't go much higher than 300°C. This means that a) the reactor operates more efficiently, and b) it avoids the risk of highly pressurized hot water, which could convert to hydrogen and explode (see: Chernobyl).
• No coolant, no reaction - in water-cooled reactors, a coolant system failure can directly lead to core meltdown (see: Fukushima). In molten salt reactors, such risks are significantly reduced. A lack of salt flow does not trigger an uncontrolled chain reaction, just as an engine cannot run without a fuel pump.
• Liquid fuel is easier to neutralize - a safety system involves draining the liquid fuel into a large, cold tank away from the reactor, where the salts quickly cool down, cannot sustain reactions, and become non-radioactive in a relatively short time.

Challenges for Metal Alloys

A mixture of sodium fluoride, zirconium, and uranium heated to over 700°C creates a highly corrosive environment. The salts cause slag formation on metal surfaces and degrade protective oxide layer.

Solution: Hastelloy® N

Hastelloy® N was specifically designed for molten salt reactors as part of the American MSRE program, the first experimental molten salt reactor built in the USA. The reactor's interior, pipes, heat exchangers, and cooling loop pumps were operational between 1964 and 1969. Although inspectors’ opinions varied, it is generally believed that Hastelloy N could withstand a full 30-year lifecycle of a commercial reactor. Hastelloy N offers:

• Corrosion resistance: Provides high resistance to corrosion in the presence of molten fluoride salts, which are a critical medium in MSR reactors. The documented corrosion rate in LiF-BeF₂ salt (the fuel salt of the MSRE reactor) at 700°C was less than 0.0025 mm per year, and 0.015 mm per year for NaBF₄-NaF coolant circuit salt heated to 600°C.
• Thermal stability: With excellent resistance to creep and deformation under heat, Hastelloy N can operate in extreme temperatures for many years without losing flexibility or cracking.

The value of Hastelloy N is supported by a scientific study published in 2021:

“Elemental analysis of the distribution of corrosion products after corrosion and analysis of EDXS, XRD, and XPS results demonstrated that Hastelloy-N performed better than any other alloy during corrosion tests, with the corrosion depth in this alloy being about 10
times lower than for alloy X750.”
~Ketan Kumar Sandhi, Jerzy A. Szpunar, Analysis of Corrosion of Hastelloy-N, Alloy X750, SS316, and SS304 in Molten Salt High-Temperature Environment

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